Lec 65: Cell Cycle Flashcards
What are the cell cycle phases?
G1 –> S –> G2 –> M
Describe G1 Phase
- cells grow in size and functional output
- mRNAs and protein synthesis
- organelle expansion
- some cells remain in G1 for extended period (G0 phase)
- prepare for S phase
- make materials required for DNA synthesis: enzymes, nucleotides, ATP
Duration: most variable (hours to years)
Describe S Phase
- duplicate each chromosome once
- 2n –> 4n
- DNA packaging must also be copied: epigenetics
Duration: 10-12 hours
Describe G2 Phase
- cells grow in preparation for division
- prepare for M phase: make materials for cell division
- tubulin, mitotic spindle proteins, AT
Duration: a few hours
Describe M Phase
- mitosis or karyokinesis
- sister chromatid (DNA) separation
- prophase, prometaphase, metaphase, anaphase, telophase
- cytokinesis
- cytoplasmic division
Duration: about 1 hour
Describe the mitotic spindle.
- The main machinery in mitosis
- Made of microtubules
- kinetochore microtubules
- astral microtubules (anchoring)
Describe Prophase
- DNA condenses to form compact chromatids
- centrosomes move to poles of nucleus
- microtubules beign to organize into spindle
Describe Prometaphase
- nuclear envelope (membrane) disintegrates
- spindle microtubules begin attaching to kinetochores on sister chromatids
Describe Metaphase
- chromosomes are aligned at the spindle equator
- sister chromatids are attached to opposite poles of spindle
Describe Anaphase
- cohesins that hold sister chromatids together are destroyed
- sister chromatids are separated and move twoards opposite poles by 2 mechanisms
- kinetochore microtubules shorten
- spindle poles move further apart from one another
Describe Telophase
- chromosomes are fully separated to opposite poles
- nuclear envelope reforms
- DNA decondenses
Describe Cytokinesis
- accomplished by progressive constriction and final scission of plasma membrane
- contractile ring (inside of cell) of actin and myosin pulls plasma membrane inward as it constricts
- spindle equator (metaphase plate) determines location
- active membrane addition is required to accommodate increase in surface area
How are cells stimulated to divide?
- mitogens
- erythropoietin
- antigen
- cytokines
- environmental stimuli initiates signal transduction cascade (turning on cyclins)
What are Cyclins?
proteins expressed at different stages in the cell cycle and direct the events that occur during that stage
How do cyclins regulate transit of cell through cell cycle?
- bind w/ cyclin-dependent kinases (Cdk) to phosphorylate targets
- different cyclin-Cdk complexes are activated/ inactivated during each phase of cell cycle
- G1-cyclin
- S-cyclin: activates DNA synthesis machinery
- M-cyclin: activates spindle regulators, histones
Describe Cell Cycle Checkpoints and what happens if a checkpoint is not satisfied
- G1 Checkpoint
- is environment favorable?
- is DNA damaged?
- G2/M Checkpoint
- is all DNA replicated properly?
- is DNA damaged?
- Meta/Anaphase Checkpoint
- are all chromosomes attached to spindle?
- Check point not satisfied?
- Cdk-inhibitor proteins turn off Cdk-cyclin complex until the problem can be dealth with
- repair or
- apoptosis
- Cdk-inhibitor proteins turn off Cdk-cyclin complex until the problem can be dealth with
Examples of diseases that arise due to cell cycle checkpoint errors/dysfunction?
Cancers
Down Syndrome
Describe what occurs when microtubules are poisoned
- prevents microtubule assembly
- cell does not make it past metaphase checkpoint
- causes cell cycle arrest and apoptosis
